Dynamic range
The main sticking point of a smaller four thirds sensor when compared to the larger APS-C format is that the dynamic range is not as wide – typically by at least 2EV.
This extra 2 stops of light captured by the APS-C format is not so much of an issue when a scene has a fairly even tone, but when shooting an overcast landscape, for instance, more highlight detail will be lost if exposing correctly for the midtones.